Method of producing hardened steel products



May 10, 1966 GRANQE ET AL 3,250,648

METHOD OF PRODUCING HARDENED STEEL PRODUCTS Filed May 14, 1963TEMPERATURE, "F

TIME /ag scale INVENTORS RAYMOND A. GRANGE and JAMES B. MITCHELL Brgmwawa A ffarney United States Patent 3,250,648 METHOD OF PRODUCINGHARDENED STEEL PRUDUCTS I Raymond A. Grange, Washington Township,Westmoreland County, and James B. Mitchell, Monroeville Borough, Pa.,assignors to United States Steel Corporation, a corporation of DelawareFiled May 14, 1963, Ser. No. 280,290

4 Claims. (Cl. 148 12.4)

This invention relates to improvements in the production of heattreatment hardened steel products and more particularly to theproduction of flat heat treatment hardened sheets.

In hardening steel products formed of steel hardenable by heat treatmentwhich steel is normally ferritic at room temperature and austenitic atelevated temperatures, warping and distortion are a severe problem inproducts of thin cross section such as sheet'product. Hardening by heattreatment involves quenching from an autenitizing temperature totransform the austenitic structure to the desired microstructure such asmartensite. Dut to the non-uniform residual stresses arising from therapid transformation of austenite to martensite, a wavy type ofdistortion results in sheet product. A variety of special techniquessuch as quenching between platens, hot straightening, austempering, ormartempering among others, have been suggested or practiced to alleviatethis problem. While minimizing the distortion to some extent, these arelargely uneconomical to practice on a commercial scale and thus, despitethe desirability of producing heat treatment hardened steel sheetproduct, it has not heretofore been produced on a large scale basis.

It is accordingly an object of this invention to produce heat treatmenthardened fiat sheets in an economical and efiicient manner.

The foregoing and further objects will be apparent from the followingspecification when read in conjunction with the attached drawingwherein:

The single figure of the drawing is a schematic representation of themethod of the invention.

We have discovered that fiat heat treatment hardened steel strip Sproduct can be continuously and rapidly produced by a combined rollingand hardening step as part of the heat treatment. Thus the strip is hotrolled slightly oversize prior to our hardening treatment and cooled toroom temperature at any desired rate. In the treatment of this inventionthe steel can be heated in any convenient manner, preferably close tothe Ac temperature in the case of hypoeutectoid steels and the Actemperature of eutectoid or hypereutectoid steels to insuresubstantially complete austenitization. During heating, scaling shouldbe minimized to insure subsequent good heat transfer between the stripand the work rolls when the strip is being worked thereby.

It is esssential that the temperature of the strip be controlled withinclose limits as it enters the work roills. Thus the work enters therolls at a temperature just barely high enough to preclude thepossibility of austenite beginning to transform until after rolling iscompleted. The opposing objectives of minimum amount of heat in thework, but no transformation of austenite, limit the temperature to thevicinity of the lower equilibrium transformation temperature (Ac of thesteel. The precise temperature range for successful hardening depends,among other things, upon the transformation characteristics of the steeland the relative size of rolls and work. Generally, a temperature rangeof from 25 F. below to about 50 F. above AC1 will be satisfactory. Withthin sheet and strip, conventional rolls used in hot rolling may producesatisfactory results by externally cooling the same by liquid sprays.Large diameter, water-cooled rolls and "ice rolls made of a highlyconductive metal such as copper alloy are all conducive to faster heatextraction and are preferred especially for thicker material wheregreater heat extraction is required. The aim in rolling is twofold.First, to reduce the work to desired thickness and,

second, to extract sufi'icient heat to insure hardening on subsequentair cooling. The amount of deformation need be only enough to insuregood contact between the surfaces of the Work and the rolls. A largeamount of deformation is, in fact, undesirable because of the heatingeffect thereof. Thus, a percentage reduction in thickness in the range 1to 20% is preferred. The essential requirement in rolling is that thetemperature of the steel fall a hundred or more degrees by rapidtransfer of heat to the rolls. As indicated in the drawing, this rapidlylowers the temperature of the work to below the nose A of its isothermaltransformation curve. Thereafter, on air cooling, the steel is made totransform in a relatively low-temperature range to a hard martensitic orbainitic product or mixtures thereof. Under optimum conditions, thisproduct will be martensite, as indicated in the drawing. Cooling in airthrough the temperature range of matensite formation (M -M is relativelyslow, which is advantageous because it minimizes distortion just as inmartempering. A flat, undistorted product results from our processingmethod because of the relatively slow and uniform cooling through the MMf range. To insure retaining maximum flatness, tension should bemaintained in the. strip as by a drive bridle 4 or the like from thestart of transformation and at least until transformation is completed.

As a specific example of our process, results obtained for modified SAE10120 high-carbon steel containing 1.26% carbon, 0.36% manganese, 0.008%phosporus, 0.025% sulphur, 0.18% silicon and 0.25% chromium are asfollows: Starting with such steel in the form of annealed 0.075" thickstrip it was first rolled to 0.050" thick, a 33 /s% reduction. Thestructure changed greatly but hardness increased only from 317 to 331D.P.H. Failure to harden appreciably in this instance is explained bythe fact that for the size and type of rolls used, the thickness was toogreat to permit sufficient heat extraction for full hardening.

However, when the cycle was repeated with 0.050" thick strip rolled thistime to 0.035" thick, a 30% reduction, hardness increases to 450 D.P.H.Although not fully hardened, the strip is now hard enough for use ascertain types of springs and cutting tools. Repeating the cycle a thirdtime and rolling from 0.035 to 0.028" thick, a 20% reduction, results inonly a small additional increase in hardness to 466 D.P.H. because heatextraction by the rolls was still insufficient to develop a martensiticmatrix. A fourth cycle, starting with 0.028" thick strip and rolling to0.025" thick, an 11% reduction, resulted in full hardening to 946 D.P.H.and a microstructure which compares favorably with that developed byconventional heat treatment. The fully hardened strip is substantiallyflat and undistorted and after tempering to any desired lower hardnesslevel is suitable for most commercial uses.

Although multi-cycle hardening treatments such as described above wouldnot normally be employed, this example was chosen to demonstrate thepossibility of developing various degrees of hardening directly on aircooling from rolling. Some of the structures developed have usefulproperties in the as-rolled condition and not even a subsequent temperneed be given. For example, a slicing knife was made by merely mountingin a handle and grinding a piece of the strip following the third stepabove described wherein the strip was hardened only to 466 D.P.H. Inthis condition, the strip could be bent, drilled, sheared, and evencold-rolled without undue difliculty and yet was hard enough for somekinds of springs and cutting tools.

The type of product most amenable to our process is the one discussedthus far, namely, strip of high-carbon,

i.e. over .9% carbon steel having a gauge less than 0.1

inch produced by rolling. However, there is no inherent limitation tosuch a product. Forging dies, for example,

can under suitable circumstances extract heatjust as do I rolls. Thus,in accordance with the principle of this invention disclosure, a varietyof forged products can be produced which are hard as forged and requireno subsequent heat treatment eXcept possibly a temper.

Thus, we have described a method of incorporating a deformation processinto the final heat treatment so as to harden steel with a minimum ofdistortion. This involves the combination of the rolling and heattreatment processes along with the requirements that steel be rolled atthe lowest temperature which preserves the austenite state duringdeformation and that the amount of reduction be small so as to maximizethe transfer of heat from the work to the rolls so that the steel israpidly cooled to below the nose of the isothermal transformation curveduring the reduction and thus insure hardening on subsequent air coolingwithout substantial distortion.

While we have shown and described several specific embodiments of ourinvention, it will be understood that these embodiments are merely forthe purpose of illustration and description and that various other formsmay be devised within the scope of our invention, as defined in theappended claims.

We claim:

1. A method of producing fiat heat treatment hardened steel sheetscomprising hot rolling such steel strip to within 1 and 20% of thedesired finished gauge, thereafter heating the hot rolled strip to justsubstantially the lowest temperature at which the microstructure isstable austem'te, reducing the gauge thereof between 1 and 20% to thedesired final gauge and simultaneously quenching the strip to atemperature below the nose of its isothermal transformation curve andthen cooling it at a rate to produce the desired microstructure'therein.

2. The method of claim 1 wherein the strip is maintained under tensionwhile transformation occurs.

3. A method of producing flat heat treatment hardened steel sheetscomprising hot rolling strip composed of steel which is hardenable byheat treatment which is ferritic at room temperature and austenitic atelevated temperatures to between 1 and 20% of the finished gaugedesired, thereafter heating the hot rolled strip to between 25 below andF. above its Ac; temperature and at which the microstructure of thesteel is stable autenite, passing said strip between a pair of rolls tosimultaneously reduce the temperature thereof at least F. to below thenose of the isothermal transformation curve of the steel and to reducethe gauge of the strip between 1 and 20% before any transformationoccurs and then air cooling said strip through the range of martensiteformation to produce a martensitic microstructure in the strip which isin a substantially flat condition. y

4. The method of claim 3 wherein the strip is maintained under tensionwhile transforming to martensite.

References (Iited by the Examiner.

UNITED STATES PATENTS DAVID L. RECK, Primary Examiner.

1. A METHOD OF PRODUCING FLAT HEAT TREATMENT HARDENED STEEL SHEETSCOMPRISING HOT ROLLING SUCH STEEL STRIP TO WITHIN 1 AND 20% OF THEDESIRED FINISHED GAUGE, THEREAFTER HEATING THE HOT ROLLED STRIP TO JUSTSUBSTANTIALLY THE LOWEST TEMPERATURE AT WHICH THE MICROSTRUCTURE ISSTABLE AUSTENITE, REDUCING THE GAUGE THEREOF BETWEEN 1 AND 20% TO THEDESIRED FINAL GAUGE AND SIMULTANEOUSLY QUENCHING THE STRIP TO ATEMPERATURE BELOW THE NOSE OF ITS ISOTHERMAL TRANSFORMATION CURVE ANDTHEN COOLING IT AT A RATE TO PRODUCE THE DESIRED MICROSTRUCTURE THEREIN.